1. Field of the Invention
The present invention generally relates to single sideband radio communications systems. More specifically, the present invention relates to a technique for transmitting analog voice signals via single sideband amplitude modulation, and digital data signals via narrowband frequency modulation, in a single trunked radio system having dedicated channel bandwidths.
2. Description of the Prior Art
In recent years, there has been a renewed interest in the research and development of a variety of narrowband communication systems. This effort has been stimulated by the severe spectrum congestion being experienced by the land mobile radio services in major metropolitan areas with the present amplitude modulation (AM) and frequency modulation (FM) systems. In the United States, 25 or 30 kHz FM systems are used throughout the VHF (very high frequency) and UHF (ultrahigh frequency) bands, while in other countries, including Great Britain, channel spacings of 12.5 kHz are also used. A further reduction in channel spacing to less than 12.5 kHz is considered necessary if future demands for spectrum are to be satisfied.
Digital voice transmission which employs linear predictive coding (LPC) and single sideband (SSB) have emerged as potential solutions to the problem of increasing spectrum congestion. For example, Carney and Linder, in their article entitled, "A Digital Mobile Radio for 5-6 kHz Channels", IEEE International Conference on Communications, Philadelphia, Pa., June 13-17, 1982, describe a 2400 bits-per-second (bps) LPC voice encoding technique for VHF-FM land mobile radios. Bit rate reduction is realized with LPC by removing redundancy through complex speech analysis and synthesis. Decreasing the high frequency content of the data, while lengthening the duration of the bit transitions, reduces the baseband data bandwidth such that a channel spacing of 5 to 6.25 kHz is feasible. However, there is a noticeable degradation in speech quality attributable to the speech encoding technique.
It has been known for many years that single sideband modulation has the advantage of a reduction in occupied bandwidth over FM systems or standard AM systems. SSB is, in reality, amplitude modulation with a carrier and one of the two sidebands suppressed, utilizing only one sideband to convey information from the transmitter to the receiver. The receiver, to demodulate the SSB signal, must recreate the suppressed carrier with the same frequency relationship to the single sideband as that of the original carrier in order to prevent distortion of the demodulated information. Voice information will sound severely distorted if the frequency error is much greater than +/-20 Hz. Furthermore, a variation in RF signal strength due to propagation conditions results in a corresponding variation in the detected audio level of the SSB signal. This causes a severe degradation of voice intelligibility.
If, however, a pilot signal is transmitted continuously with the single sideband message, and used by the receiver to track and eliminate any frequency variation imposed by fading, no frequency shift will occur in the demodulated signal. Furthermore, if the pilot signal is also used as an amplitude reference signal to automatically control the receiver gain, a constant received audio signal level may be maintained.
Numerous possibilities exist for the location of the pilot signal. The generation of a pilot carrier may simply be done by providing a controlled leakage path around the transmitter sideband filter, as described in U.S. Pat. No. 3,100,871. There, a pilot carrier signal is simultaneously transmitted with the SSB voice signal. The pilot carrier is detected by a phase locked loop, and also provides a reference signal for the operation of squelch circuitry.
An alternate SSB system approach is described in the article entitled, "Improving Spectrum Efficiency with ACSB", Communications, March 1981, by P. H. Jacobs. This VHF-SSB system utilizes a pilot tone transmitted above the voice band to provide a frequency reference for automatic frequency control (AFC), an amplitude reference for automatic gain control (AGC), and an audio subcarrier for low deviation FM tone squelch information. Syllabic amplitude companding improves the signal-to-noise ratio of the voice. Several problems remain with this approach. First, the sidebands of a digital data signal transmitted within the voice band may cause problems with frequency acquisition, since the above-band pilot tone is used as the AFC reference and must be transmitted with the data. Second, the narrow phase locked loop (PLL) required to demodulate the FM tone squelch information cannot follow the very rapid amplitude and frequency variations imposed upon a signal received in a moving vehicle. For example, multipath propagation at UHF causes fading which occurs at a rate of approximately 70 Hz at 840 MHz with a vehicle speed of 55 mi/hr. These amplitude and frequency variations cause severe distortion in the received speech signal as a consequence of the poor PLL tracking behavior.
A further problem with respect to 800 MHz radio systems is that of frequency stability. At 800 MHz, the 2 ppm (part-per-million) channel oscillators presently used in mobile radios could permit two adjacent channel mobile transmitters to drift together in frequency by as much as 3.5 kHz. With 5 or 6.25 kHz channel spacings, this much frequency error would result in a degradation in adjacent channel interference that would be intolerable. Unless very costly ultra-high stability (0.15 ppm) oscillators are utilized, adjacent narrowband channels nominally spaced 5 or 6.25 kHz apart cannot be assigned in the same area without incurring a strong likelihood of mutual interference. One solution is to abandon single channel systems in favor of structured repeater systems which afford the opportunity to impart the high frequency stability requirement of the base station to the mobile unit through the use of AFC. If the repeater incorporates multiple trunked channels, then a further improvement in spectrum utilization would be achieved. It is assumed that both voice and data must be communicated in such a trunked system, since the organization of the system is directed by data transmissions on a signalling channel.
Trunking is the automatic sharing of a block of communications channels among a large number of users. Such sharing is practical for applications in which each user requires the communications channel for only a small percentage of the time, and where few calls must be processed simultaneously. Although trunking concepts have been known and used extensively in the telephone industry and in 800 MHz FM radio systems, e.g., U.S. Pat. No. 4,012,597, little work has been done utilizing SSB trunking at 800 MHz because of the frequency stability problems referred to above. Prior communications systems requiring AFC have employed an unmodulated master reference channel having a channel bandwidth of at least twice the bandwidth of a regular voice channel, e.g., U.S. Pat. No. 4,348,772. This approach would allow ample free spectrum space for a newly activated mobile unit to search for, find, and lock to the master reference frequency. However, the utilization of this wideband reference channel approach contradicts the aforementioned goal of efficient spectrum utilization.
It is believed that there presently are no multichannel narrowband (5-6.25 KHz) UHF radio systems, AM or FM, that efficiently send and receive both analog voice and digital data signals within a dedicated single channel bandwidth. The ideal system would offer substantially the same level of performance currently enjoyed by existing FM systems--that is: high quality voice transmissions, high speed data signalling capabilities, and standard 2 ppm channel oscillator stability in the transceiver unit--while improving the spectrum efficiency with narrowband channels in the 800 MHz frequency band.